1. Field of the Invention
The present invention relates to an improved sheet feeding apparatus, and in particular, to a high speed sheet feeding apparatus which feeds sheets from a top sheet in a stack of sheets and which also employs an improved air knife device for improved separation features. In one embodiment of this invention the present invention relates to an electrophotographic machine and a top sheet feeding apparatus for use in such a machine.
2. Description of Prior Developments
In the process of electrostatographic reproduction, a light image of an original to be copied or printed is typically recorded in the form of a latent electrostatic image upon a photosensitive member, with a subsequent rendering of the latent image visible by the application of electroscopic marking particles, commonly referred to as toner. The visual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support medium, such as a sheet of plain paper. To render this toner image permanent, the image must be xe2x80x9cfixedxe2x80x9d or xe2x80x9cfusedxe2x80x9d to the paper, generally by the application of heat and pressure. The electrostatographic reproduction process is a good example of a process that involves a great deal of fast and controlled movement of sheets or paper.
With currently known high speed xerographic copy reproduction machines wherein copies can be produced at a rate in excess of several thousand copies per hour, the need for a sheet feeder to feed cut copy sheets to the machine in a rapid, dependable manner was recognized to enable full utilization of the reproduction machine""s potential copy output. In particular, for many purely duplicating operations, it is desired to feed cut copy sheets at very high speeds where multiple copies are made of an original placed on a copying platen. In addition, for many high speed copying operations, a document handler to feed documents from a stack to a copy platen of the machine in a rapid and dependable manner has also been reorganized to enable full utilization of the machine""s potential copy output. These sheet feeders must operate flawlessly to virtually eliminate the risk of damaging the sheets and generate minimum machine shutdowns due to uncorrectable misfeeds or sheet multifeeds. It is in the initial separation of the individual sheets from a stack of sheets where the greatest number of problems occurs.
Since the sheets must be handled gently, but positively to assure separation without damage through a number of cycles, a number of different types of separators have been previously suggested. These include separators, such as friction rolls or belts used for fairly positive document feeding in conjunction with a retard belt, pad, or roll to prevent multifeeds. Vacuum separators such as sniffer tubes, rocker type vacuum rolls, or vacuum feed belts have also been utilized.
While the friction roll-retard systems are generally very positive, the action of the retard member, if it acts upon the printed face, can cause smearing or partial erasure of the printed material on the document. With single sided documents if the image is against the retard mechanism, it can be smeared or erased. On the other hand, if the image is against the feed belt it smears through ink transfer and offset back to the paper. However, with documents printed on both sides the problem is compounded. Additionally, the reliable operation of friction retard feeders is highly dependent on the relative frictional properties of the paper being handled. This cannot be controlled in a document feeder.
One of the sheet feeders best known for high-speed operation is the top vacuum corrugation feeder in combination with a front air knife. In this type of system, a vacuum plenum with a plurality of friction belts that are arranged to run over the vacuum plenum is placed at the top of a stack of sheets in a supply tray. At the front of the stack, an air knife is used to inject air into the stack to separate the top sheet from the remainder of the stack. In operation, air is injected by the air knife toward the stack to separate the top sheet and the vacuum pulls the separated sheet up and acquires it. Following acquisition, the belt transport drives the sheet forward off the stack of sheets. In this type of configuration, separation of the next sheet cannot take place until the top sheet had cleared the stack. In addition, acquisition of the next sheet in the stack cannot occur until the top sheet has cleared the vacuum plenum. In this type of feeding system every operation takes place in succession or serially, and therefore the feeding of subsequent sheets cannot be started until the feeding of the previous sheet has been completed. This procedure takes time and therefore limits the potential operational speed of the sheet feeder. In such a system in order to try to increase the throughput speed, it has been proposed to activate the vacuum and the transport belts continuously. This frequently results in a difficulty in acquiring the top sheet in a stack since it must be acquired by a vacuum over which friction belts are moving. In addition, the second sheet can be prematurely acquired as the trail edge of the sheet partially clears the vacuum plenum. An overlay multifeed may occur that must be separated with another device. Thus, the inherent structure in such a system limits its potential operational speed.
A sheet feeder in answer to the above-mentioned issue is described in U.S. Pat. No. 4,451,028 in which a rear air knife vacuum corrugation feeder is disclosed that uses a moving carriage to position an air knife assembly as well as a rear vacuum assembly with respect to the trail edge of a copy sheet stack. However, the need to use a movable carriage to accommodate media of different sizes adds an added cost burden to the overall apparatus. There is also the system described in U.S. Pat. No. 4,699,369 which is an example of the use of a front air knife for a top vacuum feeder. Finally, a preferred feeding apparatus for the invention described herein is described in U.S. Pat. No. 6,624,188, in which the top sheet is acquired by a feedhead containing a plurality of corrugating ribs, separated from any other acquired sheets, and then transported to the paper path entrance.
It is a primary objective of the present invention to avoid the various disadvantages of prior art type sheet feeder devices, as described above and provide a modification to traditional air knife device designs which significantly improves air knife performance by creating high stagnation pressure area at intersheet gaps created the corrugation pattern positioned on a feedhead, thereby enhancing initial sheet separation. When combined with a multiple corrugation scheme on the feedhead, the rear jet air knife in accordance with the features of the present invention outperforms prior art type air knifes while requiring an operating pressure that is seventy-five percent (75%) less. This improvement should result in a significant reduction in unit material cost for the air source while also lowering feeder noise.
The overall objectives of this invention and other advantages over the prior art are achieved by a top sheet feeding apparatus for feeding sheets from a stack of sheets comprising a sheet stack support tray for supporting a stack of sheets, an air knife device positioned adjacent the front of the stack of sheets for applying a positive pressure to the sheet stack in order to separate the uppermost sheet in the stack from the rest of the stack, and a feedhead device including a vacuum plenum chamber positioned over the front of the sheet stack having a negative pressure applied thereto during feeding, the vacuum plenum chamber having a member in the form of a sheet corrugation pattern located in the center of its bottom surface and a translating associated with the vacuum plenum chamber to transport the sheets acquired by said vacuum plenum chamber in a forward direction out of the stack support tray, wherein the air knife device includes a pair of straight air nozzles extending from the rear portion of the air knife device.